(1) Field of the Invention
This invention relates to an optical measurement system. More particularly, it relates to a system wherein a physical change of a measured object is converted into an optical intensity change, which is measured in a position or place remote from the measured object.
(2) Description of the Prior Art
The so-called telemetering for measuring physical changes of measured objects located in distant places, e.g., changes in the position of the pointer of a measuring instrument, the level of a liquid surface, the position of an object, etc., has heretofore been well known. In conventional telemetering, a signal detected in the place of measurement is transmitted to another place after converting it into an electric signal in many cases. However, a measuring portion or a transmitting portion is susceptible to the influences of surrounding conditions such as a voltage or current, opening or closure of a power switch and a temperature fluctuation, and countermeasures against the disturbances are sometimes required. For example, from the standpoint of avoiding an explosion, it is undesirable to arrange electric applicances in the area of measurement.
In view of such drawbacks, there has also been proposed a telemetering system in which both a measuring portion and a transmitting portion are constructed of optical systems.
In a prior-art optical measurement apparatus of this type, in order to eliminate the influences of time variations on light source means and the transmitting portion, two split light beams are modulated by signals of different frequencies, whereupon they are sent to the measuring portion through an identical transmission line. In the measuring portion, one of the two split light beams is passed through a measured object, while the other light beam is superposed on the light beam having passed through the measured object. The light beams are sent via a transmission line of a return path to a signal processing portion, in which they are converted into electric signals. By exploiting the different modulation frequencies, the signals are separated. A physical change of the measured object in the measuring portion is measured on the basis of the ratio between the separated signals.
With the prior-art optical measurement apparatus, however, in a case where a single light source is utilized for preparing the two modulated light beams, a band-pass filter and an optical chopper are required. On the other hand, in a case where two light sources of different wavelengths are utilized, a problem of drifting of the light sources cannot be eliminated. Further, the processing circuit for the received optical signals requires two lock-in amplifiers. When a high-precision measurement is intended, the characteristics of the two lock-in amplifiers are required to be identifical, which makes the apparatus expensive.